The present invention generally pertains to systems and methods for modulating and demodulating ranging signals and is particularly directed to processing a modulation tone ranging signal that can be transmitted over a narrow bandwidth and simultaneously carry data.
Modulation tone ranging signals are produced by distance measurement equipment for measuring distance. Distance is determined by measuring the phase displacement of a tone signal that is transmitted over the distance to be measured. However, the tone signal frequencies that are most useful for making phase displacement measurements are much lower than those frequencies which are made available by regulatory agencies for distance measurement equipment communications. Also, useful tone signal frequencies might not have the desired electrical or electromagnetic propogation characteristics. Accordingly, a carrier signal having a frequency in the available frequency band is modulated by a tone signal having a frequency that is preferred for phase displacement measurement.
A tone ranging signal is produced for transmission in typical prior art tone ranging systems by frequency modulating (FM) or phase modulating the carrier signal with the tone signal. A significantly large bandwidth is required for both FM tone ranging signals and PM tone ranging signals. Amplitude modulation also has been considered for tone ranging signals, whereby the tone signal is mixed with the carrier signal. FIG. 1 generally depicts the frequency spectrum of the signals produced by mixing the one signal and the carrier signal. The carrier signal has a first frequency f.sub.c. The tone signal has a second frequency f.sub.sc. By mixing the tone and carrier frequencies, upper and lower sideband signals are produced at frequencies f.sub.c + f.sub.sc and f.sub.c - f.sub.sc respectively. Since the useful information is provided by the tone signals, the carrier can be suppressed and the transmitted signal can be a suppressed carrier AM sideband signal, the frequency spectrum of which is shown in FIG. 2.
Suppressed carrier AM would provide a very efficient tone ranging signal transmission method and system because it would concentrate the energy into the two sidebands, and the signal could be transmitted over a much more narrow frequency bandwidth than conventional tone ranging signals that use frequency or phase modulation. However, suppressed carrier AM would not be satisfactory for tone ranging systems because suppressed carrier AM signals would be too sensitive to phase drifts in the IF amplifier of the AM receiver. The intermediate frequency channel used for demodulating the received suppressed carrier AM signal would have to be wide enough to include both the upper and lower sideband frequencies, and the phase drift at one of the band typically would differ from the phase drift at the other end of the band. Although two different IF or measurement frequency channels could, and typically would, be used for the upper and lower sidebands, there still would be different phase drifts in the different IF or measurement frequency channels.